The invention relates to a low-pressure mercury vapor discharge lamp having a discharge vessel sealed in a gas-tight manner in which a discharge is present during operation of the lamp, said discharge vessel containing a small quantity of an alloy comprising indium, tin and zinc, which alloy forms an amalgam with mercury.
A lamp having a discharge vessel containing an amalgam comprising a metal of the group of indium, cadmium, gallium, gold, lead, tin and zinc or alloys of these metals is described in U.S. Pat. No. 4,105,910. This patent relates particularly to a low-pressure mercury vapor discharge lamp in which indium or an indium alloy with mercury as a starter amalgam is used. Such a starter amalgam is provided on a location in the lamp which is relatively hot during operation of the lamp, for example in the proximity of an electrode. When starting the lamp much mercury is released rapidly. Such a starter amalgam usually has a rich mercury content.
In addition to starter amalgams, amalgams are also known which are used in a low-pressure mercury vapor discharge lamp for stabilizing the mercury vapor pressure in the discharge vessel during operation of the lamp. The operating range of such an amalgam is determined by the temperature interval within with the mercury vapor pressure is substantially constant. This constant pressure, which occurs in the operating range, is also referred to as the plateau pressure. For low-pressure mercury vapor discharge lamps it generally holds that the light output is maximum at a mercury vapor pressure of approximately 6.times.10.sup.-3 torr. An amalgam which is used for the mercury vapor pressure control in a low-pressure mercury vapor discharge lamp thus must preferably have a plateau pressure of approximately 6.times.10.sup.-3 torr. Generally, the operating range of the known amalgams having a plateau pressure of approximately 6.times.10.sup.-3 torr is in a temperature interval below 100.degree. C.
For low-pressure mercury vapor discharge lamps such as electrodeless low-pressure mercury vapor discharge lamps having a spherical discharge vessel of, for example a diameter of approximately 110 mm and tubular low-pressure mercury vapor discharge lamps in, for example irradiation apparatus which are subjected to a relatively high load, it has been found that the light output is maximum at a mercury vapor pressure of approximately 3.times.10.sup.-3 torr. Moreover the temperature in such a lamp subjected to a relatively high load can easily rise to over 100.degree. C. during operation of the lamp. If the mercury vapor pressure for such lamps must be stabilized by means of an amalgam, an amalgam should be available which has a plateau pressure of approximately 3.times.10.sup.-3 torr and whose operating range is in a temperature interval of over 100.degree. C.